The present invention relates to a method and an apparatus for carrying out this process to determine the axis of rotation of a circular table in a multiple coordinate measuring instrument equipped with at least one tracer or feeler probe.
In the case of essentially cylindrical workpieces, measurement in a multiple coordinate measuring instrument is significantly simplified if the workpiece is rotated with respect to the tracer. With any workpiece, the rotation enhances the ease of operation, because the tracer may thereby be kept within the sight of the operator. For this purpose, the workpiece is clamped onto a circular table which may be rotated and displaced to any position within the working space of the multiple coordinate measuring instrument. The coordinates of the workpiece are related to the axis of rotation of the circular table, so that the coordinates of the axis of the rotation in the coordinate system of the instrument must be known. The determination proceeds based on the fact that, in the general case, the direction of the axis of rotation of the circular table does not coincide with any of the coordinate directions of the coordinate system of the instrument.
It is known from a reprint of a manuscript paper presented at the 8th Working Conference of the Institut fuer Produktionstechnik und Automatisierung (Institute for Production Technology and Automation) (IPA) on the "Erfahrungsaustausch Drei-Koordinaten-Messgeraete '77" ("Exchange of Experience with Three Coordinate Measuring Instruments '77") held in Stuttgart, Sept. 14-15, 1977, that the axis of rotation of a circular table may be determined by calculation from the axis of a test piece ascertained in two angular positions of the circular table. The axis of the test piece is determined by means of contact with cylindrical surfaces of the test piece both in the 0.degree. and the 180.degree. position of the circular table with the aid of the cylinder program contained in the measuring apparatus. The line of symmetry of the two axes determined is the axis of the circular table.
This method cannot be used in the case of workpieces without explicit cylindrical surfaces, and even when such surfaces are present, it depends very strongly on the quality of the surface and its geometrical dimensions. Thus, in principle, the determination of the axial direction in the case of cylinders is of an accuracy that declines with decreased heights of the cylinder.
The error of the measurement is repeated in the position rotated by 180.degree. of the workpiece and is not averaged out in the course of the determination of the line of symmetry.